Pat the boneless short ribs dry with paper towels and season with salt and black pepper
Heat a skillet over medium-high heat and add olive oil if searing the ribs
Sear the ribs for two to three minutes per side until browned then transfer to the slow cooker
Add chopped onions and minced garlic to the slow cooker spreading them evenly
In a bowl whisk together beef broth tomato paste Worcestershire sauce soy sauce balsamic vinegar brown sugar and thyme
Pour the mixture over the ribs making sure they are well coated
Add the bay leaf and cover the slow cooker with a lid
Cook on low for eight to nine hours or on high for four to five hours until the ribs are tender and fall apart easily
If the sauce is too thin remove the ribs and pour the liquid into a saucepan over medium heat
Stir in the cornstarch slurry and cook for five to seven minutes until thickened
Return the ribs to the sauce and coat well before serving
Serve hot with mashed potatoes rice or roasted vegetables